Scattered along the boulder strewn coastline of Hong Kong, eight ancient rock carvings (petroglyphs) can be found. Their beautiful geometric patterns hints of human, animal or mythical figures.
It has not been possible to establish when these rock carvings were made. The resemblance of the patterns to those on pottery and bronze vessels of the Bronze Age however suggests that they were probably carved by very early inhabitants of the region about 3000 years ago.
Based on the location of the carvings (overlooking the sea) and based on the fact that it’s creators were probably dependant on the sea for their livelihood, it is possible that these carvings were at ritual or spiritual sites serving to protect an early sea-faring population.
This specific carving was first reported by a police officer in 1970. The carved surface is 90 x 180cm and like most others in Hong Kong it is situated overlooking the bay.
The Rock carving at Big Wave Bay was declared a monument and is protected under the Antiquities and Monuments Ordinance.
A quick lesson in the geology of Big Wave Bay
What type of rock is this carving made in?
The rock formations around Big Wave Bay are part of the Che Kwu Shan Formation.
The rock types are slightly variable in this area but can be broadly classified as vitric tuffs.
But what does that mean?
All rock formations on the Earth’s surface can be classified into three groups:
1. Igneous – formed by volcanic action.
2. Sedimentary – formed when loose material is deposited in layers at the surface by water, wind, etc (e.g. sandstone, shale).
3. Metamorphic – formed when pressure or temperature changes one of the above two rock types into something new (e.g. slate, marble, schist).
With a violent volcanic past, most rock formations in Hong Kong are from the first group: Igneous.
Igneous rock can further be classified into two groups:
1. Intrusive (“plutonic”) – formed from magma under the Earth’s surface (e.g. granite).
2. Extrusive (“volcanic”) – formed from lava above the Earth’s surface.
As the rocks at the GZ was formed from volcanic material above the surface, you can therefore call it “volcanic rock”.
But we can further classify “volcanic (extrusive) rock” into three classes based on their texture and exact origin:
1. Lava flow – rocks formed when slow moving lava cools down (e.g. basalt).
2. Ash – rocks formed when the ash ejected from the volcano settles and then solidifies.
3. Pyroclastic – rocks formed from bigger rock particles ejected from the volcano.
Rocks formed from volcanic ash (number 2 above) is also called “tuffs”, and this is exactly the rocks you will see at Big Wave Bay!
But why stop there? Tuffs can further be classified into three groups based on the source of the ash:
1. Vitric – very fine ash normally of the type that flows down the side of the volcano in a pyroclastic flow (think of Pompeii in Italy).
2. Crystal – ash consisting of small crystal chips.
3. Lithic – ash comprised of pre-existing rocks coming from below the volcano.
So to summarise:
Tuff is an extrusive igneous rock that forms from the ash of an explosive volcanic eruption. This ash travels through the air and falls back to Earth in the area surrounding the volcano. If the ejected material is compacted and cemented into a rock, that rock would be called “tuff”.
Specifically, the rock formations at Big Wave Bay are:
Igneous (formed by a volcano)
-> Extrusive (formed above the surface)
-> Tuff (formed from volcanic ash)
-> Vitric (formed from fine “glass” ash flowing down the side of the volcano
Conservation of the rock carving:
The condition of the Big Wave Bay rock carving is surprisingly good. The entire carving is still intact and it is well preserved. However, the carving does face some man made hazards.
In 1983 the whole site was encased in a big glass structure. This led to a markedly different micro climate inside the structure and some serious fungal and moss growth appeared on the carving.
The glass structure was removed in 1995 and a cage-and-roof structure was built.
There were however quite a few problems with the new structure:
1. It kept rain and UV light away from the rock and thus changing the micro-climate in the rock. This caused the rock surface to dry out more than it normally would and lead to deterioration of the carving.
2. An informal slab of concrete and broken stone was laid down in front of the carving. This is most likely where any rituals or ceremonies took place, and completely ruins the overall structure and function of the site.
3. Due to the roof and some haphazard drainage structures, the ground water around the carving is now much higher than it should be and this can lead to slope degradation making the whole site unstable and prone to damage.
UPDATE: The roof and cage structure has now been removed, which is actually a good thing! Please do not touch the carving as it could easily be damaged!
Logging tasks:
Please send the answers to the following questions via e-mail or the GC messaging system to the CO.
Do not include the answers in your log.
Self study:
1. The hardness of tuff rock can vary considerably based on the origin and composition of the ash particles as well as the conditions under which it was formed.
Would you say the hardness of the carved rock is soft, medium or hard? Explain your answer.
(Keep in mind the tools and technology available 3000 years ago. Also think about how long a carving like this will last when fully exposed to the weather)
Site observations:
2. Describe the colour of the rock that the carving is made into.
3. Describe the colour of the rocks in the surrounding area.
4. In 1980 the colour of the surrounding rocks and the carved rock was exactly the same. Why do you think it is different now?
Proof of visiting GZ:
5. Look at the sign at GZ. When was this carving declared a Monument?
6. Optional: Add a photo of you or your group at the GZ to your log.
Thank you for visiting this special site in Hong Kong!
Happy caching
CX15